1. Field of the Invention
The present invention relates to a method of forming a conductive layer involving a step of forming by plating a conductive layer on an inner portion of a through-hole formed in a substrate, and a semiconductor device.
2. Description of the Related Art
Hitherto, there have been proposed various methods of forming a conductive layer on an inner portion of a through-hole (hereinafter, both a hole having an opening on one side and a hole having openings on both sides are referred to as a through-hole) formed in a substrate made of an insulating material or a substrate with an insulated surface.
By connecting a front surface to a back surface of a substrate electrically via the conductive layer formed on an inner portion of a through-hole, for example, the conductive layer is used as a through-electrode for substrate stacking. As a method of forming a conductive layer, plating is mostly used.
In a through-hole, in particular, a through-hole with a large aspect ratio (hole depth/opening diameter), a conductive layer is likely to adhere to the vicinity of an opening of the through-hole, and the opening is closed before the conductive layer is sufficiently formed by plating on a bottom of the through-hole, with the result that the film thickness of the conductive layer on the bottom of the through-hole becomes small. Therefore, there arises a problem such that electricity is hard to flow through the conductive layer in a portion where the film thickness is small.
In order to solve the above-mentioned problem, there are known a method regarding a plating liquid that uses an accelerator or an inhibitor, and a method regarding a plating current control of slowly starting a plating current value and changing the plating current value in two stages.
As one method regarding a plating liquid, there is provided a method of adding an additive such as polyethylene glycol to a plating liquid. This method prevents the opening from being closed by allowing a resin component of polyethylene glycol to be adsorbed to a portion in which an electric field is concentrated in the vicinity of the opening, thereby suppressing plating in the vicinity of the opening.
Furthermore, as one method regarding a plating current control, there is known a method in which, when a through-hole with a large aspect ratio is plated, the inner portion of the through-hole is plated at a current density smaller than that at a time of plating a flat surface. Thus, the plating liquid exchange ratio in the through-hole is enhanced with respect to a plating growth rate, and the adhesion property of plating on the bottom of the through-hole is enhanced (U.S. Patent Application Publication No. 2004/0262165A1).
However, the conventional methods have the following problems.
In the method using an additive, in the case of using a gold plating liquid that contains gold as metal, which is most stable and desirable in electrical reliability regarding a resistance and chemical reliability regarding corrosion, there arise the following problems. First, a gold cyanide plating liquid that is a cyanide bath and is a generally used gold plating liquid is a very stable solution, and hence, the effect of the additive is hardly obtained, and the opening is closed before a plating adheres completely to the bottom of the through-hole. Furthermore, the liquid state of a non-cyanide bath is very unstable, and the balance of ph or the like is likely to be lost. Thus, a decrease in precipitation efficiency, locally abnormal precipitation, and a plating defect such as burning may occur. Furthermore, the plating liquid life of the non-cyanide bath is remarkably short, and hence, the non-cyanide bath is not practical use.
In the method regarding a plating current control, when plating is performed at a recommended current density (i.e., 1/10 or less), the plating can be performed without closing the opening. However, the current density is small, and hence it takes a long period of time (i.e., 2 hours), which is a remarkably limiting condition as a tact at a time of mass-production. Hence, the method cannot be adopted. When plating is performed at a current density of about twice the value described above (i.e., ⅕) in order to set a tact time to be one hour which is a half of 2 hours, an electric field is concentrated in the opening portion, and the opening is closed before plating is completed to the bottom of the through-hole.